The general rule of thumb is a high quality 500 to 550 watt power supply with sufficient current (amps) on the 12 volt rail(s) can easily power a system with any single video card made. A high quality 700 to 750 watt power supply with sufficient current (amps) on the 12 volt rail(s) can power a system with two video cards operating in dual mode. There are a few exceptions like the new ATI Radeon HD 5XXX series cards which use less power due to their energy efficiency.

Here are the official power requirements for the brand new ATI Radeon HD 5850 and HD 5870 video cards.

ATI Radeon™ HD5850 System Requirements:

500 Watt or greater power supply with two 75W 6-pin PCI Express® power connectors recommended

600 Watt and four 6-pin connectors for ATI CrossFireX™ technology in dual mode

ATI Radeon™ HD5870 System Requirements:

500 Watt or greater power supply with two 75W 6-pin PCI Express® power connectors recommended

Before purchasing a new psu you will need to decide whether you will eventually have a pc with one or two video cards.

Corsair, PC Power & Cooling, and Seasonic are some of the brands that have a reputation for high quality power supplies that consistently earn high marks in technical reviews. They are reliable, stable, and come with a 5 year warranty. Lately we've been seeing a few other brands offering some high quality units.

I was reading this article where they tested multiple cards in single, sli, and cf setups which showed a power consumption maxing out at 664w for 2x 5870s. They used OCCT to test. However, when testing in World of Warcraft the cards only used 430w. Should I not worry too much then since I doubt I'd do anything to stress it so much and the fact that the official requirements say 600 watts?

OCCT is a test that is used to create maximum stress on a system. It is much harder on a system than gaming. According to Anandtech the OCCT test creates the largest load out of any of the games and programs they have. Since you read the technical review you know that the Intel Core i7 920 @ 3.3Ghz with two 5870's in Crossfire mode passed the test.

The power consumption charts for gaming sessions are more typical. Playing a game is not the same as performing a stress test.

NOTE - For Crossfire mode the power supply should have 40 amps on the 12 volt rail(s) and four PCI-e 6 pin power connectors for Crossfire mode.

Right now going by the final price after mail-in rebate over at newegg.com the TX850 is a better buy than the TX750 or HX650: